I'm working with an enrollment form for a prepaid card right now. In certain situations (when the user doesn't provide a social security number) the user will need to send a copy of their ID to the company before they can receive their card. In many cases the user will not do that. My theory is that they do not read through the final page and therefore don't even know that they are supposed to send their ID. I guess this is a natural thing, because usually when you submit a form and the next page does not have further form fields, you assume that you are done.

One idea I had was to send a few (2-3 in intervals) reminder emails to the user. But I also thought that there must be a way of getting the user's attention right after they complete the signup form. I was thinking about an in-page popup (like a light box) with a short warning, bur maybe there are better ways to do this?

I think you are on the right track by thinking that there's going to be a lot of abandonment around this step, for a whole bunch of reasons.

Can you collect a copy of the ID using a camera phone and send it to an email address or an SMS destination? I realize that may add a bunch of technology to the back end, but I would think the cost of creating it would be offset fairly quickly by simplifying the processing of the forms. One workflow may be:

Get the user's mobile phone number. (This also helps improve the odds that the number you get for the user is actually one where they can be reached.)

Send them a message and have them reply with a photo attached.

Associate the photo incoming from that number with the application.

Supplement the current ID validation process involving opening mail with one that's entirely at a computer.

In terms of overall flow, Hisham's suggestion is reasonable. I would take it a step further and flip-flop the choices.

Step 1: Type your SSN here, or click this button to print a form you can mail with a copy of your ID.

Step 2: Regardless of what they say above, you now collect their contact information as usual.

Step 3: If they chose the mail-in option, go to a screen that gives them a page to print with instructions to attach their copy, the mailing address for the papers, and a list of acceptable ID. The contents of this page are pretty immaterial, but it gives them a real-world token. Even if they can't/don't print it, it's a clear call back to the choice they made in step 1.